The present invention relates to a charge control circuit for a cordless telephone system including a portable unit (handset) and a base unit and more particularly to a charge control circuit which controls charge operation for a battery contained in the handset.
In a conventional cordless telephone system, a battery contained in a handset is electrically charged when the handset is set to a base unit, and charge control is conventionally carried out such that the battery is not electrically short-charged and is not over-charged. In detail, when the handset is set back to the base unit, the battery in the handset is quickly charged by a charger in the base unit and, after detection of voltage drop (.DELTA.V) caused due to the fully charged condition, trickling charge is started or charge is inhibited.
Further details of a conventional charge control of this type will be described in conjunction with FIG. 1 which illustrates only portions related to charge control in the base unit of the cordless telephone system. The conventional charge control arrangement of FIG. 1 comprises a quick charger 1, a trickling charger 2, switcher 3, a voltage drop (-.DELTA.V) detector 4, and a connection detector 5. Generally, a chargeable battery, for example, a Ni-Cd battery has a characteristic that an output voltage drops by a certain value .DELTA.V when it is fully charged. Therefore, it is possible to find the fully-charged condition by detecting the voltage drop .DELTA.V.
Further, it is assumed that a battery is contained in a handset (not shown in FIG. 1) and has a capacity of 600 mAH when fully charged. The battery is charged by the quick charger 1 when the handset is set to the base unit and, in this case, a charging current is as great as 600 mA. When a voltage drop (-.DELTA.V) is detected by the voltage drop detector 4, the trickling charger 2 is selected for trickling charge by the switcher 3. Further, when the handset is being used for speech communication, the battery is discharged at a rate of 60 mA/H.
Let it now be assumed that the battery in the handset is fully charged and the handset is used for one minute to make speech communication. When the handset is returned back to the base unit, the battery is quickly charged by the quick charger 1 via the switcher 3, and the voltage drop -.DELTA.V is detected after five minutes. In this case, the time of about five minutes is usually required to detect the voltage drop even when the battery is fully charged. This is because the rechargeable Nickel Cadmium battery has a characteristic that the voltage drop .DELTA.V occurs only after a certain period of time (about 5 minutes) from the full charge.
When the forementioned charge control is repeated, the battery is discharged by 1 mAH (60 mA.times.(1/60)) for each usage of the handset and is then charged by 50 mAH (600 mA.times.(5/60)) after each usage of the handset. Accordingly, the battery is charged by 49 mAH for each usage of the handset. In this case, after the -.DELTA.V detection, the trickling charge is continued in practice. However, the charging current in the trickling charge is so small as to compensate only for a self-discharge of the battery, and is neglected here.
Therefore, if the above charging operation is repeated several times, a charging current is added to the fully charged battery within a short period of time, that apparently means over-charging.
As described above, when the handset is used for a short period of time, the discharge and charge are repeated, and the battery is subject to over-charge even if -.DELTA.V is properly detected and the aforementioned charge control is properly completed. Therefore, the life of the battery is disadvantageously shortened, and the charger and the battery undesirably generate heat.